Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Standards Track J. Schlyter
Expires: December 16, 2012 Kirei AB
June 14, 2012
The DNS-Based Authentication of Named Entities (DANE) Transport LayerSecurity (TLS) Protocol: TLSAdraft-ietf-dane-protocol-23
Abstract
Encrypted communication on the Internet often uses Transport Level
Security (TLS), which depends on third parties to certify the keys
used. This document improves on that situation by enabling the
administrators of domain names to specify the keys used in that
domain's TLS servers. This requires matching improvements in TLS
client software, but no change in TLS server software.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 16, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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Internet-Draft DNS-Based Authentication for TLS June 20121. Introduction1.1. Background and Motivation
Applications that communicate over the Internet often need to prevent
eavesdropping, tampering, or forgery of their communications. The
Transport Layer Security (TLS) protocol provides this kind of
communications security over the Internet, using channel encryption.
The security properties of encryption systems depend strongly on the
keys that they use. If secret keys are revealed, or if public keys
can be replaced by fake keys (that is, a key not corresponding to the
entity identified in the certificate), these systems provide little
or no security.
TLS uses certificates to bind keys and names. A certificate combines
a published key with other information such as the name of the
service that uses the key, and this combination is digitally signed
by another key. Having a key in a certificate is only helpful if one
trusts the other key that signed the certificate. If that other key
was itself revealed or substituted, then its signature is worthless
in proving anything about the first key.
On the Internet, this problem has been solved for years by entities
called "Certification Authorities" (CAs). CAs protect their secret
key vigorously, while supplying their public key to the software
vendors who build TLS clients. They then sign certificates, and
supply those to TLS servers. TLS client software uses a set of these
CA keys as "trust anchors" to validate the signatures on certificates
that the client receives from TLS servers. Client software typically
allows any CA to usefully sign any other certificate.
The public CA model upon which TLS has depended is fundamentally
vulnerable because it allows any of these CAs to issue a certificate
for any domain name. A single trusted CA that betrays its trust,
either voluntarily or by providing less-than-vigorous protection for
its secrets and capabilities, can undermine the security offered by
any certificates employed with TLS. This problem arises because a
compromised CA can issue a replacement certificate that contains a
fake key. Recent experiences with compromises of CAs or their
trusted partners have led to very serious security problems, such as
the governments of multiple countries attempting to wiretap and/or
subvert major TLS-protected web sites trusted by millions of users.
The DNS Security Extensions (DNSSEC) provides a similar model that
involves trusted keys signing the information for untrusted keys.
However, DNSSEC provides three significant improvements. Keys are
tied to names in the Domain Name System (DNS), rather than to
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arbitrary identifying strings; this is more convenient for Internet
protocols. Signed keys for any domain are accessible online through
a straightforward query using the standard DNSSEC protocol, so there
is no problem distributing the signed keys. Most significantly, the
keys associated with a domain name can only be signed by a key
associated with the parent of that domain name; for example, the keys
for "example.com" can only be signed by the keys for "com", and the
keys for "com" can only be signed by the DNS root. This prevents an
untrustworthy signer from compromising anyone's keys except those in
their own subdomains. Like TLS, DNSSEC relies on public keys that
come built into the DNSSEC client software, but these keys come only
from a single root domain rather than from a multiplicity of CAs.
DNS-Based Authentication of Named Entities (DANE) offers the option
to use the DNSSEC infrastructure to store and sign keys and
certificates that are used by TLS. DANE is envisioned as a
preferable basis for binding public keys to DNS names, because the
entities that vouch for the binding of public key data to DNS names
are the same entities responsible for managing the DNS names in
question. While the resulting system still has residual security
vulnerabilities, it restricts the scope of assertions that can be
made by any entity, consistent with the naming scope imposed by the
DNS hierarchy. As a result, DANE embodies the security "principle of
least privilege" that is lacking in the current public CA model.
1.2. Securing the Association of a Domain Name with a Server's Certificate
A TLS client begins a connection by exchanging messages with a TLS
server. For many application protocols, it looks up the server's
name using the DNS to get an Internet Protocol (IP) address
associated with the name. It then begins a connection to a
particular port at that address, and sends an initial message there.
However, the client does not yet know whether an adversary is
intercepting and/or altering its communication before it reaches the
TLS server. It does not even know whether the real TLS server
associated with that domain name has ever received its initial
messages.
The first response from the server in TLS may contain a certificate.
In order for the TLS client to authenticate that it is talking to the
expected TLS server, the client must validate that this certificate
is associated with the domain name used by the client to get to the
server. Currently, the client must extract the domain name from the
certificate and must successfully validate the certificate, including
chaining to a trust anchor.
There is a different way to authenticate the association of the
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server's certificate with the intended domain name without trusting
an external CA. Given that the DNS administrator for a domain name
is authorized to give identifying information about the zone, it
makes sense to allow that administrator to also make an authoritative
binding between the domain name and a certificate that might be used
by a host at that domain name. The easiest way to do this is to use
the DNS, securing the binding with DNSSEC.
There are many use cases for such functionality. [RFC6394] lists the
ones to which the DNS RRtype in this document apply. [RFC6394] also
lists many requirements, most of which this document is believed to
meet. Section 5 covers the applicability of this document to the use
cases in detail. The protocol in this document can generally be
referred to as the "DANE TLSA" protocol. ("TLSA" does not stand for
anything; it is just the name of the RRtype.)
This document applies to both TLS [RFC5246] and DTLS [RFC6347]. In
order to make the document more readable, it mostly only talks about
"TLS", but in all cases, it means "TLS or DTLS". Although the
references in this paragraph are to TLS and DTLS version 1.2, the
DANE TLSA protocol can also be used with earlier versions of TLS and
DTLS.
This document only relates to securely associating certificates for
TLS and DTLS with host names; retrieving certificates from DNS for
other protocols is handled in other documents. For example, keys for
IPsec are covered in [RFC4025] and keys for SSH are covered in
[RFC4255].
1.3. Method For Securing Certificate Associations
A certificate association is formed from a piece of information
identifying a certificate and the domain name where the server
application runs. The combination of a trust anchor and a domain
name can also be a certificate association.
A DNS query can return multiple certificate associations, such as in
the case of a server that is changing from one certificate to another
(described in more detail in Appendix A.4).
This document only applies to PKIX [RFC5280] certificates, not
certificates of other formats.
This document defines a secure method to associate the certificate
that is obtained from the TLS server with a domain name using DNS;
the DNS information needs to be be protected by DNSSEC. Because the
certificate association was retrieved based on a DNS query, the
domain name in the query is by definition associated with the
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certificate. Note that this document does not cover how to associate
certificates with domain names for application protocols that depend
on SRV, NAPTR, and similar DNS resource records. It is expected that
future documents will cover methods for making those associations,
and those documents may or may not need to update this one.
DNSSEC, which is defined in [RFC4033], [RFC4034], and [RFC4035], uses
cryptographic keys and digital signatures to provide authentication
of DNS data. Information that is retrieved from the DNS and that is
validated using DNSSEC is thereby proved to be the authoritative
data. The DNSSEC signature needs to be validated on all responses
that use DNSSEC in order to assure the proof of origin of the data.
This document does not specify how DNSSEC validation occurs because
there are many different proposals for how a client might get
validated DNSSEC results, such as from a DNSSEC-aware resolver that
is coded in the application, from a trusted DNSSEC resolver on the
machine on which the application is running, or from a trusted DNSSEC
resolver with which the application is communicating over an
authenticated and integrity-protected channel or network. This is
described in more detail in Section 7 of [RFC4033].
This document only relates to getting the DNS information for the
certificate association securely using DNSSEC; other secure DNS
mechanisms are out of scope.
1.4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document also makes use of standard PKIX, DNSSEC, TLS, and DNS
terminology. See [RFC5280], [RFC4033], [RFC5246], and [STD13]
respectively, for these terms. In addition, terms related to TLS-
protected application services and DNS names are taken from
[RFC6125].
2. The TLSA Resource Record
The TLSA DNS resource record (RR) is used to associate a TLS server
certificate or public key with the domain name where the record is
found, thus forming a "TLSA certificate association". The semantics
of how the TLSA RR is interpreted are given later in this document.
The type value for the TLSA RR type is defined in Section 7.1.
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The TLSA RR is class independent.
The TLSA RR has no special TTL requirements.
2.1. TLSA RDATA Wire Format
The RDATA for a TLSA RR consists of a one octet certificate usage
field, a one octet selector field, a one octet matching type field
and the certificate association data field.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cert. Usage | Selector | Matching Type | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ /
/ Certificate Association Data /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.1.1. The Certificate Usage Field
A one-octet value, called "certificate usage", specifies the provided
association that will be used to match the certificate presented in
the TLS handshake. This value is defined in a new IANA registry (see
Section 7.2) in order to make it easier to add additional certificate
usages in the future. The certificate usages defined in this
document are:
0 -- Certificate usage 0 is used to specify a CA certificate, or
the public key of such a certificate, that MUST be found in any of
the PKIX certification paths for the end entity certificate given
by the server in TLS. This certificate usage is sometimes
referred to as "CA constraint" because it limits which CA can be
used to issue certificates for a given service on a host. The
presented certificate MUST pass PKIX certification path validation
and a CA certificate that matches the TLSA record MUST be included
as part of a valid certification path. Because this certificate
usage allows both trust anchors and CA certificates, the
certificate might or might not have the basicConstraints extension
present.
1 -- Certificate usage 1 is used to specify an end entity
certificate, or the public key of such a certificate, that MUST be
matched with the end entity certificate given by the server in
TLS. This certificate usage is sometimes referred to as "service
certificate constraint" because it limits which end entity
certificate can be used by a given service on a host. The target
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certificate MUST pass PKIX certification path validation and MUST
match the TLSA record.
2 -- Certificate usage 2 is used to specify a certificate, or the
public key of such a certificate, that MUST be used as the trust
anchor when validating the end entity certificate given by the
server in TLS. This certificate usage is sometimes referred to as
"trust anchor assertion" and allows a domain name administrator to
specify a new trust anchor. For example, if the domain issues its
own certificates under its own CA that is not expected to be in
the end users' collection of trust anchors. The target
certificate MUST pass PKIX certification path validation, with any
certificate matching the TLSA record considered to be a trust
anchor for this certification path validation.
3 -- Certificate usage 3 is used to specify a certificate, or the
public key of such a certificate, that MUST match the end entity
certificate given by the server in TLS. This certificate usage is
sometimes referred to as "domain-issued certificate" because it
allows for a domain name administrator to issue certificates for a
domain without involving a third-party CA. The target certificate
MUST match the TLSA record. The difference between certificate
usage 1 and certificate usage 3 is that certificate usage 1
requires that the certificate pass PKIX validation, but PKIX
validation is not tested for certificate usage 3.
The certificate usages defined in this document explicitly only apply
to PKIX-formatted certificates in DER encoding [X.690]. If TLS
allows other formats later, or if extensions to this RRtype are made
that accept other formats for certificates, those certificates will
need their own certificate usage values.
2.1.2. The Selector Field
A one-octet value, called "selector", specifies which part of the TLS
certificate presented by the server will be matched against the
association data. This value is defined in a new IANA registry (see
Section 7.3. The selectors defined in this document are:
0 -- Full certificate; the Certificate binary structure defined in
[RFC5280]
1 -- SubjectPublicKeyInfo; DER-encoded binary structure defined in
[RFC5280]
(Note that the use of "selector" in this document is completely
unrelated to the use of "selector" in DKIM [RFC6376].)
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Internet-Draft DNS-Based Authentication for TLS June 20122.1.3. The Matching Type Field
A one-octet value, called "matching type", specifies how the
certificate association is presented. This value is defined in a new
IANA registry (see Section 7.4). The types defined in this document
are:
0 -- Exact match on selected content
1 -- SHA-256 hash of selected content [RFC6234]
2 -- SHA-512 hash of selected content [RFC6234]
If the TLSA record's matching type is a hash, having the record use
the same hash algorithm that was used in the signature in the
certificate (if possible) will assist clients that support a small
number of hash algorithms.
2.1.4. The Certificate Association Data Field
The "certificate association data" to be matched. These bytes are
either raw data (that is, the full certificate or its
SubjectPublicKeyInfo, depending on the selector) for matching type 0,
or the hash of the raw data for matching types 1 and 2. The data
refers to the certificate in the association, not to the TLS ASN.1
Certificate object.
2.2. TLSA RR Presentation Format
The presentation format of the RDATA portion (as defined in
[RFC1035]) is as follows:
o The certificate usage field MUST be represented as an 8-bit
unsigned integer.
o The selector field MUST be represented as an 8-bit unsigned
integer.
o The matching type field MUST be represented as an 8-bit unsigned
integer.
o The certificate association data field MUST be represented as a
string of hexadecimal characters. Whitespace is allowed within
the string of hexadecimal characters, as described in [RFC1035].
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Internet-Draft DNS-Based Authentication for TLS June 20122.3. TLSA RR Examples
In the following examples, the domain name is formed using the rules
in Section 3.
An example of a hashed (SHA-256) association of a PKIX CA
certificate:
_443._tcp.www.example.com. IN TLSA (
0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
7983a1d16e8a410e4561cb106618e971 )
An example of a hashed (SHA-512) subject public key association of a
PKIX end entity certificate:
_443._tcp.www.example.com. IN TLSA (
1 1 2 92003ba34942dc74152e2f2c408d29ec
a5a520e7f2e06bb944f4dca346baf63c
1b177615d466f6c4b71c216a50292bd5
8c9ebdd2f74e38fe51ffd48c43326cbc )
An example of a full certificate association of a PKIX end entity
certificate:
_443._tcp.www.example.com. IN TLSA (
3 0 0 30820307308201efa003020102020... )
3. Domain Names for TLSA Certificate Associations
Unless there is a protocol-specific specification that is different
than this one, TLSA resource records are stored at a prefixed DNS
domain name. The prefix is prepared in the following manner:
1. The decimal representation of the port number on which a TLS-
based service is assumed to exist is prepended with an underscore
character ("_") to become the left-most label in the prepared
domain name. This number has no leading zeros.
2. The protocol name of the transport on which a TLS-based service
is assumed to exist is prepended with an underscore character
("_") to become the second left-most label in the prepared domain
name. The transport names defined for this protocol are "tcp",
"udp" and "sctp".
3. The base domain name is appended to the result of step 2 to
complete the prepared domain name. The base domain name is the
fully-qualified DNS domain name [RFC1035] of the TLS server, with
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the additional restriction that every label MUST meet the rules
of [RFC0952]. The latter restriction means that, if the query is
for an internationalized domain name, it MUST use the A-label
form as defined in [RFC5890].
For example, to request a TLSA resource record for an HTTP server
running TLS on port 443 at "www.example.com",
"_443._tcp.www.example.com" is used in the request. To request a
TLSA resource record for an SMTP server running the STARTTLS protocol
on port 25 at "mail.example.com", "_25._tcp.mail.example.com" is
used.
4. Use of TLSA Records in TLSSection 2.1 of this document defines the mandatory matching rules for
the data from the TLSA certificate associations and the certificates
received from the TLS server.
The TLS session that is to be set up MUST be for the specific port
number and transport name that was given in the TLSA query.
Some specifications for applications that run over TLS, such as
[RFC2818] for HTTP, require the server's certificate to have a domain
name that matches the host name expected by the client. Some
specifications such as [RFC6125] detail how to match the identity
given in a PKIX certificate with those expected by the user.
If a TLSA record has certificate usage 2, the corresponding TLS
server SHOULD send the certificate that is referenced just like it
currently sends intermediate certificates.
4.1. Usable Certificate Associations
An implementation of this protocol makes a DNS query for TLSA
records, validates these records using DNSSEC, and uses the resulting
TLSA records and validation status to modify its responses to the TLS
server.
Determining whether a TLSA RRset can be used MUST be based on the
DNSSEC validation state (as defined in [RFC4033]).
o A TLSA RRset whose DNSSEC validation state is secure MUST be used
as a certificate association for TLS unless a local policy would
prohibit the use of the specific certificate association in the
secure TLSA RRset.
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o If the DNSSEC validation state on the response to the request for
the TLSA RRset is bogus, this MUST cause TLS not to be started or,
if the TLS negotiation is already in progress, MUST cause the
connection to be aborted.
o A TLSA RRset whose DNSSEC validation state is indeterminate or
insecure cannot be used for TLS and MUST be considered unusable.
Clients which validate the DNSSEC signatures themselves MUST use
standard DNSSEC validation procedures. Clients that rely on another
entity to perform the DNSSEC signature validation MUST use a secure
mechanism between themselves and the validator. Examples of secure
transports to other hosts include TSIG [RFC2845], SIG(0) [RFC2931],
and IPsec [RFC6071]. Note that it is not sufficient to use secure
transport to a DNS resolver that does not do DNSSEC signature
validation. See Section 8.3 for more security considerations related
to external validators.
If a certificate association contains a certificate usage, selector,
or matching type that is not understood by the TLS client, that
certificate association MUST be considered unusable. If the
comparison data for a certificate is malformed, the certificate
association MUST be considered unusable.
If a certificate association contains a matching type or certificate
association data that uses a cryptographic algorithm that is
considered too weak for the TLS client's policy, the certificate
association MUST be considered unusable.
If an application receives zero usable certificate associations from
a DNS request or from its cache, it processes TLS in the normal
fashion without any input from the TLSA records. If an application
receives one or more usable certificate associations, it attempts to
match each certificate association with the TLS server's end entity
certificate until a successful match is found. During the TLS
handshake, if none of the certificate associations matches the
certificate given by the TLS server, the TLS client MUST abort the
handshake.
An attacker who is able to divert a user to a server under his
control is also likely to be able to block DNS requests from the user
or DNS responses being sent to the user. Thus, in order to achieve
any security benefit from certificate usage 0 or 1, an application
that sends a request for TLSA records needs to get either a valid
signed response containing TLSA records or verification that the
domain is insecure or indeterminate. If a request for a TLSA record
does not meet one of those two criteria but the application continues
with the TLS handshake anyway, the application has gotten no benefit
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from TLSA and SHOULD NOT make any internal or external indication
that TLSA was applied. If an application has a configuration setting
that has turned on TLSA use, or has any indication that TLSA is in
use (regardless of whether or not this is configurable), that
application either MUST NOT start a TLS connection or it MUST abort a
TLS handshake if both of the two criteria above are not met.
The application can perform the TLSA lookup before initiating the TLS
handshake, or do it during the TLS handshake: the choice is up to the
client.
5. TLSA and DANE Use Cases and Requirements
The different types of certificate associations defined in TLSA are
matched with various sections of [RFC6394]. The use cases from
Section 3 of [RFC6394] are covered in this document as follows:
3.1 CA Constraints -- Implemented using certificate usage 0.
3.2 Certificate Constraints -- Implemented using certificate usage
1.
3.3 Trust Anchor Assertion and Domain-Issued Certificates --
Implemented using certificate usages 2 and 3, respectively.
The requirements from Section 4 of [RFC6394] are covered in this
document as follows:
Multiple Ports -- The TLSA records for different application
services running on a single host can be distinguished through the
service name and port number prefixed to the host name (see
Section 3).
No Downgrade -- Section 4 specifies the conditions under which a
client can process and act upon TLSA records. Specifically, if
the DNSSEC status for the TLSA resource record set is determined
to be bogus, the TLS connection (if started) will fail.
Encapsulation -- Covered in the TLSA response semantics.
Predictability -- The appendices of this specification provide
operational considerations and implementation guidance in order to
enable application developers to form a consistent interpretation
of the recommended client behavior.
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Opportunistic Security -- If a client conformant to this
specification can reliably determine the presence of a TLSA
record, it will attempt to use this information. Conversely, if a
client can reliably determine the absence of any TLSA record, it
will fall back to processing TLS in the normal fashion. This is
discussed in Section 4.
Combination -- Multiple TLSA records can be published for a given
host name, thus enabling the client to construct multiple TLSA
certificate associations that reflect different assertions. No
support is provided to combine two TLSA certificate associations
in a single operation.
Roll-over -- TLSA records are processed in the normal manner within
the scope of DNS protocol, including the TTL expiration of the
records. This ensures that clients will not latch onto assertions
made by expired TLSA records, and will be able to transition from
using one public key or certificate usage to another.
Simple Key Management -- The SubjectPublicKeyInfo selector in the
TLSA record provides a mode that enables a domain holder to only
have to maintain a single long-lived public/private key pair
without the need to manage certificates. Appendix A outlines the
usefulness and the potential downsides to using this mode.
Minimal Dependencies -- This specification relies on DNSSEC to
protect the origin authenticity and integrity of the TLSA resource
record set. Additionally, if DNSSEC validation is not performed
on the system that wishes to use TLSA certificate bindings, this
specification requires that the "last mile" be over a secure
transport. There are no other deployment dependencies for this
approach.
Minimal Options -- The operating modes map precisely to the DANE use
cases and requirements. DNSSEC use is mandatory in that this
specification encourages applications to use only those TLSA
records that are shown to be validated.
Wild Cards -- Covered in a limited manner in the TLSA request
syntax; see Appendix A.
Redirection -- Covered in the TLSA request syntax; see Appendix A.
6. Mandatory-to-Implement Features
TLS clients conforming to this specification MUST be able to
correctly interpret TLSA records with certificate usages 0, 1, 2, and
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3. TLS clients conforming to this specification MUST be able to
compare a certificate association with a certificate from the TLS
handshake using selector types 0 and 1, and matching type 0 (no hash
used) and matching type 1 (SHA-256), and SHOULD be able to make such
comparisons with matching type 2 (SHA-512).
7. IANA Considerations
IANA is requested to make the assignments in this section. IANA
might also consider making a "DANE" section in the main IANA registry
to help developers find related registries that might be created in
the future.
In the following sections, "RFC Required" was chosen for TLSA
certificate usages and "Specification Required" for selectors and
matching types because of the amount of detail that is likely to be
needed for implementers to correctly implement new certificate usages
as compared to new selectors and matching types.
7.1. TLSA RRtype
This document uses a new DNS RR type, TLSA, whose value (52) was
allocated by IANA from the Resource Record (RR) TYPEs subregistry of
the Domain Name System (DNS) Parameters registry.
7.2. TLSA Certificate Usages
This document creates a new registry, "Certificate Usages for TLSA
Resource Records". The registry policy is "RFC Required". The
initial entries in the registry are:
Value Short description Reference
----------------------------------------------------------
0 CA constraint [This]
1 Service certificate constraint [This]
2 Trust anchor assertion [This]
3 Domain-issued certificate [This]
4-254 Unassigned
255 Private use
Applications to the registry can request specific values that have
yet to be assigned.
7.3. TLSA Selectors
This document creates a new registry, "Selectors for TLSA Resource
Records". The registry policy is "Specification Required". The
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initial entries in the registry are:
Value Short description Reference
----------------------------------------------------------
0 Full Certificate [This]
1 SubjectPublicKeyInfo [This]
2-254 Unassigned
255 Private use
Applications to the registry can request specific values that have
yet to be assigned.
7.4. TLSA Matching Types
This document creates a new registry, "Matching Types for TLSA
Resource Records". The registry policy is "Specification Required".
The initial entries in the registry are:
Value Short description Reference
--------------------------------------------------------
0 No hash used [This]
1 SHA-256 RFC 6234
2 SHA-512 RFC 6234
3-254 Unassigned
255 Private use
Applications to the registry can request specific values that have
yet to be assigned.
8. Security Considerations
The security of the DNS RRtype described in this document relies on
the security of DNSSEC to verify that the TLSA record has not been
altered.
A DNS administrator who goes rogue and changes both the A, AAAA,
and/or TLSA records for a domain name can cause the client to go to
an unauthorized server that will appear authorized, unless the client
performs PKIX certification path validation and rejects the
certificate. That administrator could probably get a certificate
issued by some CA anyway, so this is not an additional threat.
If the authentication mechanism for adding or changing TLSA data in a
zone is weaker than the authentication mechanism for changing the A
and/or AAAA records, a man-in-the-middle who can redirect traffic to
their site may be able to impersonate the attacked host in TLS if
they can use the weaker authentication mechanism. A better design
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for authenticating DNS would be to have the same level of
authentication used for all DNS additions and changes for a
particular domain name.
SSL proxies can sometimes act as a man-in-the-middle for TLS clients.
In these scenarios, the clients add a new trust anchor whose private
key is kept on the SSL proxy; the proxy intercepts TLS requests,
creates a new TLS session with the intended host, and sets up a TLS
session with the client using a certificate that chains to the trust
anchor installed in the client by the proxy. In such environments,
using TLSA records will prevent the SSL proxy from functioning as
expected because the TLS client will get a certificate association
from the DNS that will not match the certificate that the SSL proxy
uses with the client. The client, seeing the proxy's new certificate
for the supposed destination will not set up a TLS session.
Client treatment of any information included in the trust anchor is a
matter of local policy. This specification does not mandate that
such information be inspected or validated by the server's domain
name administrator.
If a server's certificate is revoked, or if an intermediate CA in a
chain between the server and a trust anchor has its certificate
revoked, a TLSA record with a certificate usage of 2 that matches the
revoked certificate would in essence override the revocation because
the client would treat that revoked certificate as a trust anchor and
thus not check its revocation status. Because of this, domain
administrators need to be responsible for being sure that the key or
certificate used in TLSA records with a certificate usage of 2 are in
fact able to be used as reliable trust anchors.
Certificates that are delivered in TLSA with certificate usage 2
fundamentally change the way the TLS server's end entity certificate
is evaluated. For example, the server's certificate might chain to
an existing CA through an intermediate CA that has certain policy
restrictions, and the certificate would not pass those restrictions
and thus normally be rejected. That intermediate CA could issue
itself a new certificate without the policy restrictions and tell its
customers to use that certificate with certificate usage 2. This in
essence allows an intermediate CA to become a trust anchor for
certificates that the end user might have expected to chain to an
existing trust anchor.
If an administrator wishes to stop using a TLSA record, the
administrator can simply remove it from the DNS. Normal clients will
stop using the TLSA record after the TTL has expired. Replay attacks
against the TLSA record are not possible after the expiration date on
the RRsig of the TLSA record that was removed.
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Generators of TLSA records should be aware that the client's full
trust of a certificate association retrieved from a TLSA record may
be a matter of local policy. While such trust is limited to the
specific domain name, protocol, and port for which the TLSA query was
made, local policy may decline to accept the certificate (for reasons
such as weak cryptography), as is also the case with PKIX trust
anchors.
8.1. Comparing DANE to Public CAs
As stated above, the security of the DNS RRtype described in this
document relies on the security of DNSSEC to verify that the TLSA
record has not been altered. This section describes where the
security of public CAs and the security of TLSA are similar and
different. This section applies equally to other security-related
DNS RRtypes such as keys for IPsec and SSH.
DNSSEC forms certificates (the binding of an identity to a key) by
combining a DNSKEY, DS, or DLV resource record with an associated
RRSIG record. These records then form a signing chain extending from
the client's trust anchors to the RR of interest.
Although the DNSSEC protocol does not enforce it, DNSKEYs are often
marked with a SEP flag indicating whether the key is a Zone Signing
Key (ZSK) or a Key Signing Key (KSK). ZSKs protect records in the
zone (including DS and DLV records), and KSKs protect ZSK DNSKEY
records. This allows KSKs to be stored offline.
The TLSA RRtype allows keys from the DNSSEC PKI hierarchy to
authenticate keys wrapped in PKIX certificates for a particular host
name, protocol, and port.
With the exception of the DLV RRtype, all of these certificates
constrain the keys they identify to names that are within the zone
signing the certificate. In order for a domain's DLV resource
records to be honored, the domain must be configured as a DLV domain,
and the domain's DNSKEYs must be configured as trust anchors or be
authentic [RFC5074].
8.1.1. Risk of Key Compromise
The risk that a given certificate that has a valid signing chain is
fake is related to the number of keys that can contribute to the
validation of the certificate, the quality of protection each private
key receives, the value of each key to an attacker, and the value of
falsifying the certificate.
DNSSEC allows any set of domains to be configured as trust anchors
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and/or DLVs, but most clients are likely to use the root zone as its
only trust anchor. Also, because a given DNSKEY can only sign
resource records for that zone, the number of private keys capable of
compromising a given TLSA resource record is limited to the number of
zones between the TLSA resource record and the nearest trust anchor,
plus any configured DLV domains. Typically, this will be six keys,
half of which will be KSKs.
PKIX only describes how to validate a certificate based on a client-
chosen set of trust anchors, but says nothing about how many trust
anchors to use or how they should be constrained. As currently
deployed, most PKIX clients use a large number of trust anchors
provided with the client or operating system software. These trust
anchors are selected carefully, but with a desire for broad
interoperability. The trust anchors and CA certificates for public
CAs rarely have name constraints applied.
A combination of technical protections, process controls, and
personnel experience contribute to the quality of security that keys
receive.
o The security surrounding DNSSEC DNSKEYs varies significantly. The
KSK/ZSK split allows the KSK to be stored offline and protected
more carefully than the ZSK, but not all domains do so. The
security applied to a zone's DNSKEYs should be proportional to the
value of the domain, but that is difficult to estimate. For
example, the root DNSKEY has protections and controls comparable
to or exceeding that of public CAs. On the other end of the
spectrum, small domains might provide no more protection to their
keys than they do to their other data.
o The security surrounding public CAs also varies. However, due to
financial incentives and standards imposed by clients for
acceptance into their trust anchor stores, CAs generally employ
security experts and protect their keys carefully, though highly-
public compromises have occurred.
8.1.2. Impact of Key Compromise
The impact of a key compromise differs significantly between the two
models.
o DNSKEYs are inherently limited in what they can sign, so a
compromise of the DNSKEY for "example.com" provides no avenue of
attack against "example.org". Even the impact of a compromise of
.com's DNSKEY, while considerable, would be limited to .com
domains. Only the compromise of the root DNSKEY would have the
equivalent impact of an unconstrained public CA.
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o Public CAs are not typically constrained in what names they can
sign, and therefore a compromise of even one CA allows the
attacker to generate a certificate for any name in the DNS. A
domain holder can get a certificate from any willing CA, or even
multiple CAs simultaneously, making it impossible for a client to
determine whether the certificate it is validating is legitimate
or fraudulent.
Because a TLSA certificate association is constrained to its
associated name, protocol, and port, the PKIX certificate is
similarly constrained, even if its public CAs signing the certificate
(if any) are not.
8.1.3. Detection of Key Compromise
If a key is compromised, rapid and reliable detection is important in
order to limit the impact of the compromise. In this regard, neither
model prevents an attacker from near-invisibly attacking their
victim, provided that the necessary keys are compromised.
If a public CA is compromised, only the victim will see the
fraudulent certificate, as there is typically no publicly-accessible
directory of all the certificates issued by a CA that can be
inspected. DNS resource records are typically published publicly.
However, the attacker could also allow the uncompromised records to
be published to the Internet as usual but provide a compromised DNS
view to the victim to achieve the same effect.
8.1.4. Spoofing Hostnames
Some CAs implement technical controls to ensure that certificates are
not issued to domains with names similar to popular & prone-to-attack
domains. Of course, an attacker can attempt to circumvent this
restriction by finding a CA willing to issue the certificate anyway.
However, by using DNSSEC and TLSA, the attacker can circumvent this
check completely.
8.2. DNS Caching
Implementations of this protocol rely heavily on the DNS, and are
thus prone to security attacks based on the deliberate mis-
association of TLSA records and DNS names. Implementations need to
be cautious in assuming the continuing validity of an association
between a TLSA record and a DNS name.
In particular, implementations SHOULD rely on their DNS resolver for
confirmation of an association between a TLSA record and a DNS name,
rather than caching the result of previous domain name lookups. Many
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platforms already can cache domain name lookups locally when
appropriate, and they SHOULD be configured to do so. It is proper
for these lookups to be cached, however, only when the TTL (Time To
Live) information reported by the DNS makes it likely that the cached
information will remain useful.
If implementations cache the results of domain name lookups in order
to achieve a performance improvement, they MUST observe the TTL
information reported by DNS. Implementations that fail to follow
this rule could be spoofed or have access denied when a previously-
accessed server's TLSA record changes, such as during a certificate
rollover.
8.3. External DNSSEC Validators
Due to a lack of DNSSEC support in the most commonly-deployed stub
resolvers today, some ISPs have begun checking DNSSEC in the
recursive resolvers they provide to their customers, setting the AD
flag as appropriate. DNSSEC-aware clients could use that data,
ignoring the fact that the DNSSEC data has been validated externally.
Because there is typically no authentication of the recursive
resolver or integrity protection of the the data and AD flag between
the client and the recursive resolver, this can be trivially spoofed
by an attacker.
However, even with secure communications between a host and the
external validating resolver, there is a risk that the external
validator could become compromised. Nothing prevents a compromised
external DNSSEC validator from claiming that all the records it
provides are secure, even if the data is falsified, unless the client
checks the DNSSEC data itself (rendering the the external validator
unnecessary).
For this reason, DNSSEC validation is best performed on-host, even
when a secure path to an external validator is available.
9. Acknowledgements
Many of the ideas in this document have been discussed over many
years. More recently, the ideas have been discussed by the authors
and others in a more focused fashion. In particular, some of the
ideas and words here originated with Paul Vixie, Dan Kaminsky, Jeff
Hodges, Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam
Langley, Ben Laurie, Ilari Liusvaara, Ondrej Mikle, Scott Schmit,
Ondrej Sury, Richard Barnes, Jim Schaad, Stephen Farrell, Suresh
Krishnaswamy, Peter Palfrader, Pieter Lexis, Wouter Wijngaards, John
Gilmore, and Murray Kucherawy.
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Authentication of Named Entities (DANE)", RFC 6394,
October 2011.
[X.690] "Recommendation ITU-T X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", 2002.
Appendix A. Operational Considerations for Deploying TLSA RecordsA.1. Creating TLSA Records
When creating TLSA records, care must be taken to avoid
misconfigurations. Section 4 of this document states that a TLSA
RRset whose validation state is secure MUST be used. This means that
the existence of such a RRset effectively disables other forms of
name and path validation. A misconfigured TLSA RRset will
effectively disable access to the TLS server for all conforming
clients, and this document does not provide any means of making a
gradual transition to using TLSA.
When creating TLSA records with certificate usage 0 (CA Certificate)
or usage 2 (Trust Anchor), one needs to understand the implications
when choosing between selector type 0 (full certificate) and 1
(SubjectPublicKeyInfo). A careful choice is required because
different methods for building trust chains are used by different TLS
clients. The following outlines the cases that one ought to be aware
of and discusses the implications of the choice of selector type.
Certificate usage 2 is not affected by the different types of chain
building when the end entity certificate is the same as the trust
anchor certificate.
A.1.1. Ambiguities and Corner Cases When TLS Clients Build Trust Chains
TLS clients can implement their own chain-building code rather than
rely on the chain presented by the TLS server. This means that,
except for the end entity certificate, any certificate presented in
the suggested chain might or might not be present in the final chain
built by the client.
Certificates that the client can use to replace certificates from the
original chain include:
o Client's trust anchors
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o Certificates cached locally
o Certificates retrieved from a URI listed in an Authority
Information Access X.509v3 extension
CAs frequently reissue certificates with different validity periods,
signature algorithms (such as an different hash algorithm in the
signature algorithm), CA key pairs (such as for a cross-certificate),
or PKIX extensions where the public key and subject remain the same.
These reissued certificates are the certificates TLS client can use
in place of an original certificate.
Clients are known to exchange or remove certificates that could cause
TLSA certificate associations that rely on the full certificate to
fail. For example:
o The client considers the signature algorithm of a certificate to
no longer be sufficiently secure
o The client might not have an associated root certificate in its
trust store and instead uses a cross-certificate with an identical
subject and public key.
A.1.2. Choosing a Selector Type
In this section, "false-negative failure" means that a client will
not accept the TLSA certificate association for a certificate
designated by DNS administrator. Also, "false-positive acceptance"
means that the client accepts a TLSA association for a certificate
that is not designated by the DNS administrator.
A.1.2.1. Selector Type 0 (Full Certificate)
The "Full certificate" selector provides the most precise
specification of a TLSA certificate association, capturing all fields
of the PKIX certificate. For a DNS administrator, the best course to
avoid false-negative failures in the client when using this selector
is:
1. If a CA issued a replacement certificate, don't associate to CA
certificates that have a signature algorithm with a hash that is
considered weak by local policy.
2. Determine how common client applications process the TLSA
certificate association using a fresh client installation, that
is, with the local certificate cache empty.
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A SubjectPublicKeyInfo selector gives greater flexibility in avoiding
some false-negative failures caused by trust-chain-building
algorithms used in clients.
One specific use-case ought to be noted: creating a TLSA certificate
association to CA certificate I1 that directly signed end entity
certificate S1 of the server. The case can be illustrated by
following graph:
+----+ +----+
| I1 | | I2 |
+----+ +----+
| |
v v
+----+ +----+
| S1 | | S1 |
+----+ +----+
Certificate chain sent by A different validation path
server in TLS handshake built by the TLS client
I2 is a reissued version of CA certificate I1 (that is, it has a
different hash in its signature algorithm).
In the above scenario, both certificates I1 and I2 that sign S1 need
to have identical SubjectPublicKeyInfos because the key used to sign
S1 is fixed. An association to SubjectPublicKeyInfo (selector type
1) will always succeed in such a case, but an association with a full
certificate (selector type 0) might not work due to a false-negative
failure.
The attack surface is a bit broader compared to "full certificate"
selector: the DNS administrator might unintentionally specify an
association that would lead to false-positive acceptance.
o The administrator must know or trust that the CA does not engage
in bad practices, such as not sharing the key of I1 for unrelated
CA certificates leading to trust-chain redirect. If possible, the
administrator ought to review all CA certificates that have the
same SPKI.
o The administrator ought to understand whether some PKIX extension
may adversely affect security of the association. If possible,
administrators ought to review all CA certificates that share the
SubjectPublicKeyInfo.
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o The administrator ought to understand that any CA could, in the
future, issue a certificate that contains the same
SubjectPublicKeyInfo. Therefore, new chains can crop up in the
future without any warning.
Using the SubjectPublicKeyInfo selector for association with a
certificate in a chain above I1 needs to be decided on a case-by-case
basis: there are too many possibilities based on the issuing CA's
practices. Unless the full implications of such an association are
understood by the administrator, using selector type 0 is a better
option from a security perspective.
A.2. Provisioning TLSA Records in DNSA.2.1. Provisioning TLSA Records with Aliases
The TLSA resource record is not special in the DNS; it acts exactly
like any other RRtype where the queried name has one or more labels
prefixed to the base name, such as the SRV RRtype [RFC2782]. This
affects the way that the TLSA resource record is used when aliasing
in the DNS.
Note that the IETF sometimes adds new types of aliasing in the DNS.
If that happens in the future, those aliases might affect TLSA
records, hopefully in a good way.
A.2.1.1. Provisioning TLSA Records with CNAME Records
Using CNAME to alias in DNS only aliases from the exact name given,
not any zones below the given name. For example, assume that a zone
file has only the following:
sub1.example.com. IN CNAME sub2.example.com.
In this case, a request for the A record at "bottom.sub1.example.com"
would not return any records because the CNAME given only aliases the
name given. Assume, instead, the zone file has the following:
sub3.example.com. IN CNAME sub4.example.com.
bottom.sub3.example.com. IN CNAME bottom.sub4.example.com.
In this case, a request for the A record at bottom.sub3.example.com
would in fact return whatever value for the A record exists at
bottom.sub4.example.com.
Application implementations and full-service resolvers request DNS
records using libraries that automatically follow CNAME (and DNAME)
aliasing. This allows hosts to put TLSA records in their own zones
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or to use CNAME to do redirection.
If the owner of the original domain wants a TLSA record for the same,
they simply enter it under the defined prefix:
; No TLSA record in target domain
;
sub5.example.com. IN CNAME sub6.example.com.
_443._tcp.sub5.example.com. IN TLSA 1 1 1 308202c5308201ab...
sub6.example.com. IN A 192.0.2.1
sub6.example.com. IN AAAA 2001:db8::1
If the owner of the original domain wants to have the target domain
host the TLSA record, the original domain uses a CNAME record:
; TLSA record for original domain has CNAME to target domain
;
sub5.example.com. IN CNAME sub6.example.com.
_443._tcp.sub5.example.com. IN CNAME _443._tcp.sub6.example.com.
sub6.example.com. IN A 192.0.2.1
sub6.example.com. IN AAAA 2001:db8::1
_443._tcp.sub6.example.com. IN TLSA 1 1 1 536a570ac49d9ba4...
Note that it is acceptable for both the original domain and the
target domain to have TLSA records, but the two records are
unrelated. Consider the following:
; TLSA record in both the original and target domain
;
sub5.example.com. IN CNAME sub6.example.com.
_443._tcp.sub5.example.com. IN TLSA 1 1 1 308202c5308201ab...
sub6.example.com. IN A 192.0.2.1
sub6.example.com. IN AAAA 2001:db8::1
_443._tcp.sub6.example.com. IN TLSA 1 1 1 ac49d9ba4570ac49...
In this example, someone looking for the TLSA record for
sub5.example.com would always get the record whose value starts
"308202c5308201ab"; the TLSA record whose value starts
"ac49d9ba4570ac49" would only be sought by someone who is looking for
the TLSA record for sub6.example.com, and never for sub5.example.com.
Note that deploying different certificates for multiple services
located at a shared TLS listener often requires the use of TLS SNI
(Server Name Indication) [RFC6066].
Note that these methods use the normal method for DNS aliasing using
CNAME: the DNS client requests the record type that they actually
want.
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Using DNAME records allows a zone owner to alias an entire subtree of
names below the name that has the DNAME. This allows the wholesale
aliasing of prefixed records such as those used by TLSA, SRV, and so
on without aliasing the name itself. However, because DNAME can only
be used for subtrees of a base name, it is rarely used to alias
individual hosts that might also be running TLS.
; TLSA record in target domain, visible in original domain via DNAME
;
sub5.example.com. IN CNAME sub6.example.com.
_tcp.sub5.example.com. IN DNAME _tcp.sub6.example.com.
sub6.example.com. IN A 192.0.2.1
sub6.example.com. IN AAAA 2001:db8::1
_443._tcp.sub6.example.com. IN TLSA 1 1 1 536a570ac49d9ba4...
A.2.1.3. Provisioning TLSA Records with Wildcards
Wildcards are generally not terribly useful for RRtypes that require
prefixing because one can only wildcard at a layer below the host
name. For example, if one wants to have the same TLSA record for
every TCP port for www.example.com, the result might be:
*._tcp.www.example.com. IN TLSA 1 1 1 5c1502a6549c423b...
This is possibly useful in some scenarios where the same service is
offered on many ports or the same certificate and/or key is used for
all services on a host. Note that the domain being searched for is
not necessarily related to the domain name found in the certificate,
so a certificate with a wildcard in it is not searched for using a
wildcard in the search request.
A.3. Securing the Last Hop
As described in Section 4, an application processing TLSA records
must know the DNSSEC validity of those records. There are many ways
for the application to determine this securely, and this
specification does not mandate any single method.
Some common methods for an application to know the DNSSEC validity of
TLSA records include:
o The application can have its own DNS resolver and DNSSEC
validation stack.
o The application can communicate through a trusted channel (such as
requests to the operating system under which the application is
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running) to a local DNS resolver that does DNSSEC validation.
o The application can communicate through a secured channel (such as
requests running over TLS, IPsec, TSIG or SIG(0)) to a non-local
DNS resolver that does DNSSEC validation.
o The application can communicate through a secured channel (such as
requests running over TLS, IPsec, TSIG or SIG(0)) to a non-local
DNS resolver that does not do DNSSEC validation, but gets
responses through a secured channel from a different DNS resolver
that does DNSSEC validation.
A.4. Handling Certificate Rollover
Certificate rollover is handled in much the same was as for rolling
DNSSEC zone signing keys using the pre-publish key rollover method
[RFC4641]. Suppose example.com has a single TLSA record for a TLS
service on TCP port 990:
_990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
To start the rollover process, obtain or generate the new certificate
or SubjectPublicKeyInfo to be used after the rollover and generate
the new TLSA record. Add that record alongside the old one:
_990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
_990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...
After the new records have propagated to the authoritative
nameservers and the TTL of the old record has expired, switch to the
new certificate on the TLS server. Once this has occurred, the old
TLSA record can be removed:
_990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...
This completes the certificate rollover.
Appendix B. Pseudocode for Using TLSA
This appendix describes the interactions given earlier in this
specification in pseudocode format. If the steps below disagree with
the text earlier in the document, the steps earlier in the document
ought to be considered correct and this text incorrect.
Note that this pseudocode is more strict than the normative text.
For instance, it forces an order on the evaluation of criteria which
is not mandatory from the normative text.
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